In-depth analysis of variable scope in JavaScript_javascript skills

Before learning JavaScript variable scope, we should clarify a few points:

•JavaScript’s variable scope is based on its unique scope chain.

• JavaScript does not have block-level scope.

• Variables declared in a function are defined throughout the function.

1. JavaScript scope chain
First look at the following code:

Copy code The code is as follows:

Observe the code alert(rain); . JavaScript first checks whether the variable rain is defined in the inner function. If it is defined, the rain variable in the inner function is used. If the rain variable is not defined in the inner function, JavaScript will continue to check whether the rain variable is defined in the rainman function. In this code, the rain variable is not defined in the rainman function body, so the JavaScript engine will continue to look up (the global object) to see if rain is defined; in the global object, we have defined rain = 1, so the final result will pop up '1'.

Scope chain: When JavaScript needs to query a variable If the two objects are not defined, the search will continue, and so on.

The above code involves three scope chain objects, in order: inner, rainman, and window.

2. Within the function body, local variables have a higher priority than global variables with the same name.

Copy code The code is as follows:

3. JavaScript does not have block-level scope.

This is also the more flexible part of JavaScript compared to other languages.

Look carefully at the following code, you will find that the scopes of variables i, j, and k are the same, and they are global in the entire rain function body.

Copy code The code is as follows:

4. In function Declared variables are defined throughout the function.

First observe this code:

Copy the code The code is as follows:

The above code shows that the variable x can be used in the entire rain function body and can be reassigned. Due to this rule, "unbelievable" results will be produced, observe the following code.

Copy code The code is as follows:

is because the local variable x in the function rain is defined throughout the function body (var x= 'rain-man', declared), so it is declared throughout the function rain The global variable x with the same name is hidden in the rain function body. The reason why 'undefined' pops up here is because when alert(x) is executed for the first time, the local variable x has not yet been initialized.

So the rain function above is equivalent to the following function:

Copy the code The code is as follows:

function rain(){ var x; alert( x ); x = 'rain-man'; alert( x );}

5. Unused var Variables defined by keywords are global variables.

Copy code The code is as follows:

This is also a common mistake among JavaScript newbies, leaving many global variables unintentionally.

6. Global variables are all attributes of the window object

Copy the code The code is as follows:

Is equivalent to the following code

Copy code The code is as follows: